1. Field of the Invention
The present invention relates to the atomisation of liquid fuels for combustion. More specifically, this invention is directed to mechanical atomisers and particularly to a novel and improved pressure jet atomiser of the centrifugal swirl type. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
As is well known, liquid fuels are subjected to atomisation in order to cause a greatly increased surface area of the liquid to be exposed to the air; i.e., to promote intimate mixing of the liquid fuel and the gases needed to support combustion. In effecting the atomisation of a liquid fuel, force is applied to the liquid to set it in motion. The moving liquid should preferably be ejected into a combustion chamber as an attenuated film which is disrupted into filaments. The filaments thereafter disintegrate into small particles or fragments which, as a result of the surface tension of the liquid itself, assume spherical droplet form whereby an atomised spray results. The breakup of the attenuated film into filaments may be accomplished by the application of outside forces or as a result of kinetic energy in the liquid derived from the initial force which promoted its flow.
A number of different methods of atomisation are known in the prior art. The prior art atomisers include steam atomisers, air atomisers, Weir atomisers; also known as drooling atomisers; mechanical atomisers; wherein a rotating cup-type element is employed; and pressure jet atomisers. Pressure jet atomisers may be either of the plain orifice or centrifugal swirl type. A pressure jet atomiser of the centrifugal swirl type is disclosed in U.S. Pat. No. 2,373,707.
Pressure jet atomisers of the centrifugal swirl type are used extensively in the oil guns of boilers for steam generators of the types employed by electrical utilities and used on large marine vessels. In the typical prior art centrifugal swirl type atomiser, the liquid fuel under pressure is fed into a swirl or vortex chamber through tangentially disposed slots. The rotating mass of liquid in the swirl chamber is forced toward a discharge orifice; the discharge orifice having a diameter which is small when compared to the diameter of the swirl chamber. As a result of the translational force which urges the liquid axially toward the discharge orifice and the centrifugal force which urges the liquid against the restricting boundary walls of the swirl chamber and orifice, the liquid is ejected from the orifice as a rapidly thinning divergently conical film. This film is, in theory, inherently unstable and will break up into spherical droplets; these spherical droplets defining the desired spray.
Prior art centrifugal swirl type pressure jet atomisers have been characterized by a number of inherent deficiencies. The most significant of these deficiencies has resided in an inability of the prior art devices to achieve the desired atomisation efficiency over a wide range of fuel flows. Thus, in typical prior art centrifugal swirl type atomisers, the liquid film which emerges from the discharge orifice increases in thickness in an inverse ratio with flow. As the fuel film thickness increases, of course, there is greater resistance to rupture. As a closely allied problem, prior art swirl-type atomisers have also been characterized by instability with the spray angle varying substantially with pressure. Restated, the prior art swirl-type atomisers have been characterized by an unstable spray pattern; i.e., a spray pattern which tends to wander.